1. Field of the Invention
Embodiments of the present invention generally relate to methods for forming semiconductor devices. More particularly, embodiments of the present invention generally relate to methods for pre-cleaning a substrate surface prior to forming a metal silicide on a semiconductor substrate.
2. Description of the Related Art
Metal gates or contacts typically include a doped silicon surface, one or more barrier layers, one or more liner layers and bulk metal to complete the gate structure. The cleanliness of the substrate surface between layers is critical for reducing contact resistance and hence, optimal device performance. For logic devices, the contact is usually a silicide, such as nickel silicide (NiSi), cobalt silicide (CoSi2), or titanium silicide (TiSi2). Nickel silicide is becoming more popular for smaller geometries, e.g., geometries having aspect ratios of about 10:1 or smaller, because NiSi is widely available and has a lower resistivity and lower contact resistance compared to other metal silicides.
In a typical fabrication process, a pre-clean process is performed before the metal silicide is formed on a substrate in one vacuum environment. Prior to the metal silicide fabrication process, the substrate may be transferred among different vacuum environment to perform a different processing step. As a result, the substrate can be subjected to oxidative conditions during the transfer, accumulating native oxides or contaminants on the substrate surface prior to the metal silicide fabrication process. Excess native oxide accumulation or contaminants may adversely affect the nucleation capability of the metal elements to adhere on the substrate surface to form the metal silicide layer. Poor nucleation of the metal elements on the interface may result in poor adhesion of the metal silicide layer to the silicon substrate surface, thereby resulting in peeling and particle flakes. Furthermore, poor adhesion at the interface may also result in undesired high contact resistance, thereby resulting in poor electrical properties of the device characteristics. In addition, poor nucleation of the metal elements adhered on the silicon substrate surface may further result in different stoichiometric ratios of metal elements to silicon elements formed in the resultant metal silicide layer, which may impact not only on the electrical performance of the devices, but also on the integration of the deposition of the conductive contact material subsequently formed thereon.
Therefore, there is a need for an improved pre-cleaning process for removing native oxides from the substrate surface prior to a metal silicide fabrication process.